Researchers have long been searching for RF multipole systems with axially superimposed electric potential profiles for the manipulation of ions in different ways, for example guiding the ions through sections of instruments (“ion guides”), even against flows of gas molecules. The ions may be manipulated, for example, for generation of longitudinal oscillations of the ions, for production of finely focused ion beams, for reactions between ions of opposite polarity, and/or for fragmentation and thermalization of ions. Ideally, axially superimposed electric potential profiles may be switched between different profile shapes. In addition to temporarily storing and thermalizing ions, such multipole systems should be able to, for example, fragment the ions via collisions with collision gas molecules and subsequently or simultaneously transport the fragmented ions to an exit at an end of the multipole system.
A “two-dimensional multipole field” may be defined as a field generated by alternatively applying two different voltages to two or more pairs of pole rods included in a multipole system. The voltages may be DC voltages or AC voltages. Effective radially repelling pseudoforces for ions, however, typically only occur with RF voltages.
Pole rods of a multipole system may be cylindrical sheath segments, rectangular plates, round rods or hyperbolic rods, depending on the desired quality of the multipole field. An ideal multipole field is generated in the vicinity of an axis, but typically only extends radially up to the pole rods when the pole rods have a certain hyperbolic shape. The multipole field may deviate for other shapes more or less strongly from the ideal multipole field, the greater the distance from the axis, which particularly affects the repulsive forces of the pseudopotential.
The radially repulsive pseudoforce produced by the pseudopotentials is typically strongest for RF quadrupole electrode systems having two pairs of pole rods. The ions in such quadrupole systems are trapped in a virtual tube, figuratively speaking, by repulsive pseudoforces which increase radially in each direction. The ions may move freely in the axial direction without an axial potential gradient; i.e., the ions are not trapped in the axial direction. The ions may oscillate freely about the axis with so-called “secular oscillations” under high vacuum conditions. The ion oscillations may be damped by collisions, however, in a medium vacuum, where the ions collect on the axis. The aforesaid process may be referred to as “collision focusing” or “thermalization” of the ions. Quadrupole systems with a linear potential drop along the axis correspond to sloping tubes where the content flows in one direction under the influence of the slope. They therefore form an “ion chute”. Multipole systems with larger numbers of rod pairs, such as hexapole or octopole rod systems, have lower radially repulsive pseudoforces, but also form such tubes for ions. Axial potential profiles in such systems may also transmit or trap ions as a function of the shape of the profile.
A longitudinal electric field may be superimposed by producing a quadrupole electrode system out of four resistance wires, across each of which a DC voltage drop is generated in the same direction. The wires carry a relatively high RF voltage to generate the quadrupole RF field because the largest voltage drop occurs in the immediate vicinity of each wire. Resistance of each wire should not be particularly high because, otherwise, the RF alternating voltage cannot propagate quickly enough along the wires. Relatively small DC voltage drops therefore are typically generated along each wire. It may also be difficult to generate desired profiles of the DC electric field which are not simply linear voltage gradients along the axis. Ions may also be able to easily escape because the pseudopotential barrier between the wires is relatively low.
A longitudinal electric field may also be superimposed using a quadrupole system having a large number of parallel wires mounted so as to reproduce four hyperbolic surfaces of an ideal quadrupole system. Such a hyperbolic quadrupole system reproduced with wires was developed approximately 50 years ago by the research group of Wolfgang Paul. While quadrupole systems are difficult to produce and may be imprecise, they do provide a simple way of generating an axial DC field by generating voltage drops across the wires.
Other ion storage systems which have an electrically switched forward feed are disclosed in U.S. Pat. No. 5,572,035 to Franzen. The '035 patent discloses, for example, a system that includes two helically coiled conductors in a shape of a double helix, and operated by being connected to two phases of an RF voltage. The '035 patent also discloses a system including coaxial rings to which the phases of an RF AC voltage are alternately connected. Both systems may be operated to generate an axial feed of the ions. The double helix may be made from resistance wires across which a DC voltage drop is generated. The individual rings of the ring system may be supplied with a DC potential that changes from ring to ring. This may also be used to tailor desired shapes of axial potential profiles.
U.S. Pat. No. 5,847,386 to Thomson et al. discloses methods for generating an axial voltage drop in quadrupole round rod systems. In one embodiment, the quadrupole system is divided up into a large number of axially separated segments. The '386 patent also discloses penetrating resistance layers carrying a DC voltage drop with RF fields as DC potentials are introduced into the quadrupole rod system from the outside by surrounding electrodes.
U.S. Pat. No. 7,164,125 to Franzen et al. discloses generating axial DC potential profiles by insulated resistance layers.
Each of the aforesaid techniques, however, has various drawbacks. The disclosed systems, for example, may not provide ideal potential profiles, may be difficult to manufacture, and/or may not be switchable or adjustable.
In addition to the generation of axial DC voltage profiles in multipole systems, the generation of axial pseudopotential profiles is also of great interest. If one disregards very weak pseudopotential gradients in conical multipole rod systems, only pseudopotential barriers at the ends of multipole systems have been described up to now.
There is a need in the art therefore for elongated ion cells with electrically adjustable shapes of radial and axial distributions of DC potentials and pseudopotentials.